1. Field of the Invention
The present invention relates to an electronic flash device for a photographic camera, and, more particularly, to an electronic flash device with law specific power consumption for a camera.
2. Description Related to the Prior Art
Cameras and lens-fitted film units have built-in electronic flashes for convenience of taking pictures indoors or under law ambient subject brightness. Such an electronic flash device has a need of charging a main capacitor up to a specified charged level of voltage. When the main capacitor is completely charged up, a neon lamp connected to both terminals of the main capacitor is energized or turned on to emit light for providing an indication that the electronic flash device is ready to flash. An electronic flash device equipped with a light emitting diode (which is hereinafter referred to as LED) as used for an indicator instead of the neon lamp has been proposed in, for example, Japanese Unexamined Patent Publication No. 8-115796 filed by the same applicant of this application and placed on the market, the LED needs a build-up voltage of 1.8 or higher to turn on and emit light. However, electromotive force of a battery that is usually used in cameras and lens-fitted film units is about 1.5 volts which is too low to energize directly the LED. The electronic flash device disclosed in the above mentioned publication energizes the LED with a voltage that is provided by a blocking oscillator that is constituted by an oscillating transistor and an oscillating transformer and well known in various form to those in the art. Reference is made to FIG. 6 for the purpose of providing a brief background that will enhance an understanding of the operation of a circuit of the electronic flash device disclosed in the above-mentioned publication.
Referring to FIG. 6, the electronic flash device includes a blocking oscillator that is comprised of an oscillating transistor 60 and an oscillating transformer 61. The oscillating transistor 60 repeatedly increases/decreases a primary current I1 which flows through a primary winding 61a of the oscillating transformer 61 so as to generate an electromotive force and a counter electromotive force across secondary and third windings 61b and 61c, respectively. When an electromotive force builds up, a main capacitor 63 is charged with a secondary current I2 which flows through a rectifier diode 62 from the secondary winding 61b. While a charging switch 64 remains turned on or closed, a battery 66 can start to supply current I0 through a resistor 65 and the third winding 61c of the oscillating transformer 61 to a base of the transistor 60, as a result of which the transistor 60 is turned conductive to admit the primary current I1 to flow therthrough. This causes the secondary and third windings 61b and 61c to produce the secondary and third currents 12 and 13, respectively. These currents I2 and I3 are added to the current I0 supplied originally from the battery 66 with the result of increasing the base current of the oscillating transistor 60, which leads to a further increase in the primary current I1, so that the base current reaches a peak current instantaneously due to a further increase in the secondary current I2. On the other hand, when the primary current I1 reaches a peak level and then stops increasing, each winding, 61a, 61b and 61c generates a counter electromotive force which is opposite in direction to the electromotive force. The counter electromotive force across the secondary and third windings 61b and 61c cause a reduction in the base current of the oscillating transistor 60, which results in a reduction in the primary current I1 correspondingly. In consequence, there occurs a further increase in the counter electromotive force, which leads an instantaneous reduction in the base current to a bottom level. As a result, when the counter electromotive force disappears, the oscillating transistor 60 is brought into conductive, so as to repeat the same operation.
As described above, the LED 67 for providing an indication of completion of charging a main capacitor is connected to both ends of the tertiary winding 61c which gives ON/OFF oscillation to the transistor 60 by amplifying the amplitude of a base current of the transistor 60 with a current which is generated as a current 13 when electromotive force is generated across the tertiary winding 61c or as a current (-I3) opposite in direction to the current I3 when counter electromotive force is generated across the tertiary winding 61c. In order to energize LED 67 to emit light when the main capacitor 63 attains a specified charged voltage, the utilization is made of a potential present at one of the opposite ends of the tertiary winding 61c that changes in proportional to a charged voltage of the main capacitor 63.
In the case where, although it has no concern in installation of a light emitting diode, the tertiary winding is used to control, increase or reduce, the base current of the oscillating transistor by connecting the tertiary winding to the light emitting diode, the current I0 supplied from the battery is not supplied as a base current to the base transistor and is, however, cancelled out by the current (-I3) when counter electromotive force is generated across the tertiary winding. That is to say, the battery wastes power by letting the current I0 to flow. Besides the current I0 is rather large as the resister used in the circuit through which the current I0 flows has a relatively low resistance such as 200 ohms. Accordingly, the electronic flash device described above unnecessarily consumes electric power and causes the battery to waste easily its power. In the case where a current is directly supplied as a base current to the oscillating transistor from the battery by way of a resister having resistance of about 200 ohms in place of amplifying the amplitude of a base current of the oscillating transistor through the tertiary winding, the oscillating transistor remains turned ON and does not implement oscillation. Otherwise, in the case where a current is supplied as a base current to the oscillation transistor from the battery through a resister having a high resistance of, for example, 1 K ohms, while the oscillating transistor implements oscillation, it is turned nonconductive with counter electromotive force generated across the secondary winding. In consequence, when the counter electromotive force across the secondary winding becomes weak due to a rise in the charged voltage of the main capacitor to a somewhat high level, the oscillating transistor remains conductive due to a reduction in amplitude of the base current, as a result of which charging stops before the main capacitor attains a specified charged voltage and the oscillating transistor is continuously supplied with a current from the battery.